Conventional radio base stations are located adjacent to the antenna in a small but at the base of the antenna tower. Finding suitable sites can be a challenge because of the footprint required for the hut, the need for structural reinforcement of rooftops and the availability of both primary and backup power sources. In contrast to the conventional base stations, a recent approach has been the introduction of a so-called distributed radio base station architecture. In this case, the functionality of the radio base station is divided or distributed, wherein the radio frequency transceivers are separated from the rest of the base station and relocated next to their associated antennas such that the antennas are driven directly with minimal transmission power loss. The digital base band data is transported between the base band processing located in the central base station enclosure and the remotely located radio frequency transceivers over a flexible, loss-free optical fibre. Consequently, it is possible to remotely locate the radio frequency transceiver a significant distance from the main base station such that a single, central base station or base band “hotel” can centrally serve a large number of these remote radio frequency transceivers.
The Common Public Radio Interface (CPRI) is an initiative to define a publicly available specification that standardizes the protocol interface between the radio equipment control (REC) node and the radio equipment (RE) node in wireless distributed base stations. This allows interoperability of equipment from different vendors and preserves the software investment made by the wireless service providers. CPRI allows the use of a distributed architecture where base stations, containing the REC, are connected to the remote radio heads via lossless fibre links that carry the CPRI data. The architecture reduces cost for service providers because only the remote radio heads containing the RE need to be situated in environmentally challenging locations. The base stations can be centrally located in less challenging locations where footprint climate and availability of power are more easily managed. Typically, the CPRI links are between a REC node and a RE node, or between two RE nodes in a chain or cascade configuration. Various arrangements of RE nodes and a REC in a distributed base station topology are illustrated in FIG. 1. Radio equipment nodes are either directly or indirectly connected to the radio equipment control node. In order to reduce the number of fibers to a radio equipment control node, a plurality of radio equipment nodes can be connected to a common CPRI concentrator.
A general outline of the relation between a radio equipment control node and a radio equipment node is illustrated in FIG. 2. The two nodes are in communication via the previously mentioned CPRI interface, which includes a plurality of logical connections or data flows. These logical connections encompass a synchronization data flow for synchronization and timing information between nodes, a control and management (C&M) data flow for control data used for call processing and management information for operation, administration and maintenance of the CPRI link and nodes, and CPRI layer 1 control data. In addition, user plane data is transported in the form of IQ data. Several IQ data flows are sent via one physical CPRI link, wherein each IQ data flow reflects the data of one antenna for one carrier.
There is a growing interest in building large base band hotels and have distributed radio heads, using the aforementioned CPRI for transmission in between radio equipment control nodes and radio equipment nodes, as well as for transmission between radio equipment nodes. There is also a growing interest in using multiple antennas per sector e.g. eight antenna branches or more per radio equipment. The combination presents a tough challenge.
One problem with the above described base band hotel is the need for a very high speed interface between the base band hotel e.g. radio equipment control node and the remote radios e.g. radio equipment nodes. With future aggregation of LTE carriers, e.g. a combination of 8 or more antennas per sector and up to 50 MHz bandwidth, the resulting CPRI rate may reach up to 25 Gbps per radio head e.g. radio equipment node, and result in significant physical layer processing in the base band e.g. radio equipment control node.
As an example, changing from two to eight antennas per radio equipment node, would result in an increase in interface bit rates from 2 Gbps to 8 Gbps/radio equipment. In addition, changing bandwidth from 20 MHz to 50 MHz would cause an increase in CPRI interface traffic from 8 Gbps/RE to 20 Gbps/RE. Trying to uphold this type of interface bit rate is both costly and computationally challenging, not to mention the resource waste involved in maintaining the possibility to support those interface bit rates even when the traffic demand is low for a specific time period.
Based on the above mentioned problems, there is a need for an improved utilization of the radio resources in distributed radio base stations, whilst still maintaining at least a lowest acceptable service layer at all times.